Vegetated marine and freshwater habitats are being increasingly lost around the world. Habitat restoration is a critical step for conserving these valuable habitats, but new approaches are needed to increase restoration success and ensure their survival. We investigated interactions between plants and bivalves through a review and analysis of 491 studies, determined the effects, mechanisms and key environmental variables involved in and driving positive and negative interactions, and produced guidelines for integrating positive interactions into restoration efforts in different habitats. Fifty per cent of all interactions (both correlative and experimental studies) were positive. These were predominant between epifaunal bivalves and plants in all habitats, and between infaunal bivalves and plants in subtidal habitats. Plants primarily promoted bivalve survival and abundance by providing substrate and shelter, while bivalves promoted plant growth and survival by stabilizing and fertilizing the sediment, and reducing water turbidity. The prevalence of positive interactions increased with water temperature in subtidal habitats, but decreased with water temperature in intertidal habitats. The subset of studies conducted in a restoration context also showed mostly positive interactions. Twenty‐five per cent of all interactions were negative, and these were predominant between plants and infaunal bivalves in intertidal habitats, except sulphide‐metabolizing bivalves, which facilitated plant survival. Interactions involving non‐native species were also mostly negative. Synthesis and applications. Promoting facilitative interactions through plant–bivalve co‐restoration can increase restoration success. The prevalence of positive interactions depends on habitat and environmental conditions such as temperature, and was especially important in subtidal habitats (involving both infaunal and epifaunal bivalves) and in intertidal habitats (involving only epifaunal bivalves). Thus sites and species for co‐restoration must be carefully chosen to maximize the chances of success. If done properly, co‐restoration could increase initial survival, persistence and resilience of foundation species, and promote the recovery of associated biodiversity and ecosystem services.

Propagule dispersal is an integral part of the life cycle of seagrasses; important for colonising unvegetated areas and increasing their spatial distribution. However, to understand recruitment success, seed dispersal and survival in habitats of different complexity remains to be quantified. We tested the single and synergistic effects of three commonly distributed ecosystem engineers—eelgrass ( Zostera marina ), oysters ( Magellana gigas ) and blue mussels ( Mytilus edulis )—on trapping of Z . marina seeds in a hydraulic flume under currents. Our results suggest that seed retention increases with habitat complexity and further reveal insights into the underlying mechanisms. In eelgrass canopy, trapping occurred mostly through direct blocking of a seed’s pathway, while trapping in bivalve patches was mainly related to altered hydrodynamics in the lee side, i.e. behind each specimen. With increasing flow velocity (24–30 cm s -1 in eelgrass canopy, 18–30 cm s -1 in bivalve patches), modifications of the sediment surface through increased turbulence and erosive processes became more important and resulted in high seed trapping rates. Furthermore, we show that while monospecific patches of seagrass and bivalves had different trapping optima depending on flow velocities, intermixing resulted in consistently high trapping rates throughout the investigated hydrodynamic gradient. Our results highlight the importance of positive interactions among ecosystem engineers for seed retention and patch emergence in eelgrass.

Foundation species host diverse associated communities by ameliorating environmental stress. The strength of this facilitative effect can be highly dependent on the underlying biotic and abiotic context. We investigated community level patterns of macrofauna associated with and adjacent to the marine foundation species eelgrass (Zostera marina) along a hydrodynamic stress gradient. We could demonstrate that the relative importance of this foundation species for its infaunal community increases with environmental variables associated with increasing hydrodynamic stress (depth, sand ripples formation, sediment grain size and organic content). Faunal assemblages in proximity to the Zostera patch edges, however, showed no (infauna) or negative (epifauna) response to hydrodynamic stress. Our study highlights that the facilitative outcome of a foundation species is conditional to the faunal assemblage in question and can be highly variable even between positions within the habitat.

Marine ecosystem engineers such as seagrasses and bivalves create important coastal habitats sustaining high biodiversity and ecosystem services. Restoring these habitats is difficult due to the importance of feedback mechanisms that can require largescale efforts to ensure success. Incorporating facilitative interactions could increase the feasibility and success of small-scale restoration efforts, which would limit pressure on donor sites and reduce costs and time associated with restoration. Here, we tested two methods for providing facilitation in small-scale eelgrass (Zostera marina) restoration plots across northern Europe:(1) co-restoration with blue mussels (Mytilus edulis, M. trossulus); and (2) the use of biodegradable establishment structures (BESEs). Eelgrass-mussel co-restoration showed promise in aquaria, where eelgrass growth was nearly twice as high in treatments with medium and high mussel densities than in treatments without mussels. However, this did not translate to higher shoot length or shoot densities in subsequent field experiments. Rather, hydrodynamic exposure limited both eelgrass and mussel survival, especially in the most exposed sites. The use of BESEs showed more potential in enabling small-scale restoration success: they effectively enhanced eelgrass survival and reduced mussel loss, and showed potential for enabling mussel recruitment in one site. However, eelgrass planted in BESE plots along with mussels had a lower survival rate than eelgrass planted in BESE plots without mussels. Overall, we show that though co-restoration did not work at small scales, facilitation by using artificial structures (BESEs) can increase early eelgrass survival and success of small-scale eelgrass and bivalve restoration.

Facilitative interactions between co-occurring species sustain diverse communities and constitute a vital functional component of coastal marine ecosystems. In seagrass ecosystems, facilitation ensures the survival and resilience of this important habitat. As seagrass meadows are in decline, innovative restoration strategies incorporating facilitative interactions could open new avenues in marine restoration. Here, we investigated the interactions between eelgrass Zostera marina and the Baltic clam Macoma balthica, and tested whether clams could enhance early survival and biomass increase of transplanted eelgrass shoots in the northern Baltic Sea. We measured eelgrass responses to differing densities of clams, as well as porewater ammonium (NH4+) and phosphate (PO43-) concentrations in field and aquarium experiments. Overall, survival of transplanted plots was high, independent of clam density. Specifically, we found that clams facilitated eelgrass above- and below-ground biomass in low porewater nutrient conditions, potentially through nutrient release, but inhibited growth in high-nutrient conditions, particularly where clams were added at high densities. Our results show the important role of infaunal bivalves for nutrient fluxes within seagrass meadows. Most notably, we highlight the importance of considering and testing context- and density-dependency when studying interspecific interactions, as clams could both benefit and hamper Zostera biomass increase. This becomes particularly crucial when incorporating such interactions in a restoration context.

The rocky intertidal communities of Ireland contain a mix of cold- and warm-adapted species, however the spatial distribution of these communities has not been investigated in a systematic way. Based on a benthic community dataset collected in 2003 at 63 sites, several statistical analyses were combined with the aims of (i) detecting groups of similar communities and their spatial arrangement, (ii) relating these groups to environmental factors and (iii) identifying the species that drive the different community groups. Sørensen's index suggested two marine community groups, one of the east and south-east (termed ‘east’) and the other in the west, south-west and north (termed ‘west’). A second partition based on combined wave exposure and sea surface chlorophyll comprised four groups, as did a further partition based on combined sea surface and air temperatures. The spatial arrangement of wave height plus chlorophyll conditions agreed reasonably well with the binary marine community partition, but the temperature partition did not. The ‘east’ community appeared to be associated with low wave height and chlorophyll conditions. The species that were most influential to the ‘east’ community were Balanus crenatus, Austrominius modestus and Fucus vesiculosus. The ‘west’ sites were associated with high wave height/low chlorophyll (with some variation in this due to local shelter) and the species Paracentrotus lividus, Chthamalus stellatus, Alaria esculenta and Himanthalia elongata. A longitudinal pattern rather than one associated with latitude was evident in this marine community and local drivers rather than temperature clines appeared most important for the dominant community patterns.

Habitat forming ecosystem engineers play critical roles in structuring coastal seascapes. Many ecosystem engineers, such as seagrasses and epifaunal bivalves, are known to have positive effects on sediment stability and increase coastal protection and ecosystem resilience. Others, such as bioturbating infaunal bivalves, may instead destabilize sediment. However, despite the common co-occurrence of seagrasses and bivalves in coastal seascapes, little is known of their combined effects on sediment dynamics. Here, we used wave flumes to compare sediment dynamics in monospecific and multispecific treatments of eelgrass, Zostera marina, and associated bivalves (infaunal Limecola balthica, infaunal Cerastoderma edule, epifaunal Magellana gigas) under a range of wave exposures. Eelgrass reduced bedload erosion rates by 25-50%, with digital elevation models indicating that eelgrass affected the sediment micro-bathymetry by decreasing surface roughness and ripple sizes. Effects of bivalves on sediment mobilization were species-specific; L. balthica reduced erosion by 25%, C. edule increased erosion by 40%, while M. gigas had little effect. Importantly, eelgrass modified the impacts of bivalves: the destabilizing effects of C. edule vanished in the presence of eelgrass, while we found positive additive effects of eelgrass and L. balthica on sediment stabilization and potential for mutual anchoring. Such interspecific interactions are likely relevant for habitat patch emergence and resilience to extreme wave conditions. In light of future climate scenarios where increasing storm frequency and wave exposure threaten coastal ecosystems, our results add a mechanistic understanding of sediment dynamics and interactions between ecosystem engineers, with relevance for management and conservation.

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